[TenTec] Orion AM ALC

[Dr. Gerald N. Johnson]

When you average an audio signal amplified with an AC amplifier, e.g. 
capacitor or transformer coupled there is no DC component. It probably 
is symmetrical about that average value. So when it gets to the 
modulator it has to have even excursions up and down from 100% carrier. 
If you modulate negative more than 100% you introduce clipping 
distortion and splatter. If you modulate positive more than 100% you 
often introduce distortion from clipping when the output stage can't 
prove the required peak power. If you try to introduce asymmetrical 
modulation (more up than down) you are asking the transmitter to produce 
more average power than the rated carrier power. That's DC offset. Audio 
states are hardly ever DC coupled because you want to limit the low 
frequency components that don't add to communications. Usually for 
communications a low cut of 300 Hz is common. That minimizes the 
modulation from trombones, tubas, cellos, and basses but communication 
is not music.

In vacuum tube or solid state transmitters (the biggest I've worked on 
ran 250KW carrier, 1 MW peak) the only place you can cause asymmetrical 
modulation is at the modulation stage, in many cases the PA. AT 250KW we 
had enough trouble keeping tubes without trying some upward modulation 
scheme demanding even more RF power.

Symmetrical 100% modulation is clean and can be low distortion. Going 
above 100% negative is always a source of splatter from cutting off the 
carrier abruptly. Going above 100% positive modulation can be a source 
of splatter from exceeding the peak power capability of the transmitter, 
whether low level modulation with linear or high level modulation. These 
make good reasons to stick with 100% maximum modulation, which is 
supported by FCC rules on high quality signals.

Yes, the power is IN the audio, that's why SSB works so much better than 
AM for communications with weak or strong signals. AM remains popular 
for consumer applications because receivers are simple and receiver 
tuning is not critical. AM on CB is a downer because the FCC limits on 
modulation causes CB makers to seriously limit modulation which makes 
the radios ineffective at communication. That's why amplified 
microphones are popular in CB. I used to check the 50.4 calling 
frequency on 6m when it was open, most of the time I'd find carriers on 
50.4 not long after hearing many SSB signals between 50.125 and 50.2, 
but I'd copy hundreds of SSB signals before the band got good enough to 
copy modulation on 50.4 AM. It was vivid demonstration of the benefits 
of SSB. SSB benefits by putting all the transmitter power in one side 
band. SSB benefits by limiting the noise bandwidth of the receiver to 
that one audio sideband. Those two give SSB a great advantage over AM, 
at least 8 dB less transmitter power for the same S/N at the speaker.

73, Jerry, K0CQ

On 6/13/2010 8:10 AM, Rob Atkinson wrote:
> Hi Jerry,
>
> Kindly expand upon your statement re "DC offset."  You lost me with that one.
>
> I think what you are writing below has to do with vacuum tube plate
> modulated rigs?  A number of asymmetric peak limiter processors for AM
> have come to market since the early 1970s.  Yes, some of the older
> broadcast rigs had a hard time with them.
>
> Merle:  Too bad Ten Tec assumes everyone wants to be limited to 100%
> positive and negative.   The power is in the audio.
>
> 73
>
> Rob
> K5UJ
>
>